Lubricating oil composition for internal combustion engine

ABSTRACT

A lubricating oil composition for an internal combustion engine including: (A) a lubricant base oil having a kinematic viscosity at 100° C. of 2 to 8 mm 2 /s and having an aromatic content of no more than 10 mass %; (B) a metallic detergent including: (B1) a metallic detergent overbased with calcium carbonate; and (B2) a metallic detergent overbased with magnesium carbonate; and (C) a molybdenum sulfide dithiocarbamate or a molybdenum oxysulfide dithiocarbamate, wherein the composition has, on the basis of the total mass of the composition, a calcium content of no more than 1500 mass ppm, a magnesium content of no less than 300 mass ppm, and a molybdenum content of no less than 600 mass ppm; and the composition has an HTHS viscosity at 150° C. of no more than 2.7 mPa·s.

FIELD

The present invention relates to lubricating oil compositions forinternal combustion engines.

BACKGROUND

Recently, it has been proposed to replace a conventional naturallyaspirated engine with an engine having less displacement and equippedwith a turbocharger (turbocharged downsized engine), so as to improvefuel efficiency of automobile engines, especially of automobile gasolineengines. Turbocharged downsized engines make it possible to reducedisplacement while maintaining engine power, and thus to improve fuelefficiency, owning to the turbocharger.

Disadvantageously, turbocharged downsized engines may suffer aphenomenon that ignition occurs in a cylinder earlier than an expectedtiming (LSPI: Low Speed Pre-Ignition), when its torque is increased at alow rotation speed. LSPI leads to increase of energy loss, and thus torestriction on fuel efficiency improvement and low-speed torqueimprovement. Engine oil is suspected to have an influence on occurrenceof LSPI. It has been reported that the calcium content in engine oilpromotes occurrence of LSPI.

CITATION LIST Patent Literature

-   Patent Literature 1: WO 2015/114920 A1-   Patent Literature 2: JP H7-316577 A-   Patent Literature 3: JP 2014-152301 A-   Patent Literature 4: JP 2015-143304 A-   Patent Literature 5: JP 2015-140354 A-   Patent Literature 6: JP 5727701 B2-   Patent Literature 7: WO 2015/111746 A1

Non Patent Literature

-   Non-Patent Literature 1: Takeuchi, K.; Ito, Y; Fujimoto, K.,    “Investigations of Engine Oil Effect on Abnormal Combustion in    Turbocharged Direct Injection—Spark Ignition Engines (Part    1)—Preventing or Contributing to Low-Speed Pre-Ignition through    Effects of Engine Oil Additives”, Proceedings of JSAE Annual    Congress 2012, No. 70-12, pp. 1-4, 20125101 (May 25, 2012, JSAE    Annual Congress (Spring)).-   Non-Patent Literature 2: Fujimoto, K.; Yamashita, M.; Kaneko, T.;    Takeuchi, K.; Ito, Y.; Matsuda, H., “Investigations of Engine Oil    Effect on Abnormal Combustion in Turbocharged Direct Injection—Spark    ignition Engines (Second Report)—Correlation between Auto-Ignition    Temperature of Engine Oil and Low-Speed Pre-Ignition Frequency”,    Proceedings of JSAE Annual Congress 2012, No. 70-12, pp. 5-8,    20125109 (May 25, 2012, JSAE Annual Congress (Spring)).-   on-Patent Literature 3: Okada, Y.; Miyashita, S.; Yaguchi, H.;    Izumi, Y; Aoki, F., “Study of LSPI Occurring Mechanism from    Deposit”, Proceedings of JSAE Annual Congress 2014, No. 94-14, pp.    11-16, 20145633 (Oct. 22, 2014, JSAE Annual Congress (Autumn)).-   Non-Patent Literature 4: Seki, Y; Negoro, K.; Sato, Y; Matsuura, K.;    Nishi, M.; Iida, N., “An. Analysis of the mechanism of Pre-ignition    in turbo-charged Direct injection spark ignition engines”,    Proceedings of JSAE Annual Congress 2014, No. 94-14, pp. 23-28,    20145825 (Oct. 22, 2014, JSAE Annual Congress (Autumn)).-   Non-Patent Literature 5: Fujimoto, K.; Yamashita, M.; Hirano, S.;    Kato, K.; Watanabe, I.; Ito, K., “Engine Oil Development for    Preventing Pre-Ignition in Turbocharged Gasoline Engine”, SAE    Int. J. Fuels Lubr. 2014, 7(3), 869-874, doi:10.4271/2014-01-2785.

SUMMARY Technical Problem

A Ca content in engine oil is derived from a metallic detergent, whichis an additive to keep an engine clean, Thus, reducing the Ca content soas to suppress LSPI in turn results in insufficient engine detergency.

An object of the present invention is to provide a lubricating oilcomposition for an internal combustion engine having improved LSPIsuppression performance while having both engine detergency and fuelefficiency.

Solution to Problem

A first aspect of the present invention is a lubricating oil compositionfor an internal combustion engine comprising: (A) a lubricant base oilhaving a kinematic viscosity at 100° C. of 2 to 8 mm²/s and having anaromatic content of no more than 10 mass %; (B) a metallic detergentcomprising: (B1) a metallic detergent overbased with calcium carbonate;and (B2) a metallic detergent overbased with magnesium carbonate; and(C) a molybdenum sulfide dithiocarbamate or a molybdenum oxysulfidedithiocarbamate, wherein the composition has a calcium content of nomore than 1500 mass ppm on the basis of the total mass of thecomposition; the composition has a magnesium content of no less than 300mass ppm on the basis of the total mass of the composition; thecomposition has a molybdenum content of no less than 600 mass ppm on thebasis of the total mass of the composition; and the composition has anHTHS viscosity at 150° C. of no more than 2.7 mPa·s.

In this specification, “kinematic viscosity at 100° C.” means kinematicviscosity at 100° C., which is specified by ASTM D-445; and “HTHSviscosity at 150° C.” means viscosity at a high shear rate and hightemperature at 150° C., which is specified by ASTM D4683.

A second aspect of the present invention is a method for suppressingLSPI of an internal combustion engine, the method comprising: operatingan internal combustion engine, while lubricating a cylinder of theinternal combustion engine by means of the lubricating oil compositionaccording to the first aspect of the present invention.

Advantageous Effects of Invention

According to the first aspect of the present invention, a lubricatingoil composition for an internal combustion engine having improved LSPIsuppression performance while having both engine detergency and fuelefficiency can be provided.

The method for suppressing LSPI of an internal combustion engineaccording to the second aspect of the present invention uses thelubricating oil composition according to the first aspect of the presentinvention, which can effectively suppress LSPI of an internal combustionengine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a scatter diagram in which occurrence frequency of LSPI in anengine test is plotted against the DSC (10 atm) auto-ignition point ofan engine oil sample used in the engine test.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will be described hereinafter. Expression “A to B”concerning numeral values A and B means “no less than A and no more thanB” unless otherwise specified. In such expression, if a unit is addedonly to the numeral value B, the same unit is applied to the numeralvalue A as well A word “or” means a logical sum unless otherwisespecified.

<(A) Lubricant Base Oil>

In the lubricating oil composition of the present invention, a lubricantbase oil having a kinematic viscosity at 100° C. of 2 to 8 mm²/s andhaving an aromatic content of no more than. 10 mass % (hereinafter maybe referred to as “lubricant base oil of the present embodiment”) isused as a base oil.

Examples of the lubricant base oil of the present embodiment includeparaffinic mineral oils, normal paraffinic base oils, isoparaffinic baseoils, and mixtures thereof having a kinematic viscosity at 100° C. of 2to 8 mm²/s and having an aromatic content of no more than 10 mass %,which are obtained by refining lubricant oil fractions that are obtainedby atmospheric distillation and/or vacuum distillation of crude oils,through one or more refining processes selected from solventdeasphalting, solvent extraction, hydrocracking, solvent dewaxing,catalytic dewaxing, hydrorefining, sulfuric acid washing, claytreatment, etc.

Preferred examples of the lubricant base oil of the present embodimentinclude a base oil, a raw material of which is any of the following baseoils (1) to (8), and which is obtained by recovering lubricant oilfractions derived from refining, through a predetermined refiningmethod, oil of the raw material and/or lubricant oil fractions recoveredfrom the oil of the raw material:

(1) a distillate obtained by atmospheric distillation of paraffin basecrude oils and/or mixed base crude oils;

(2) a distillate obtained by vacuum distillation of residual oils ofparaffin base crude oils and/or mixed base crude oils (WVGO);

(3) a wax obtained through a lubricant oil dewaxing process (slack waxetc.) and/or synthetic wax obtained through a gas to liquid (GTL)process or the like (Fischer-Tropsch wax, GTL wax, etc.);

(4) a mixed oil of at least one selected from the base oils (1) to (3)and/or a mild hydrocracked oil of the mixed oil;

(5) a mixed oil of at least two selected from the base oils (1) to (4);

(6) a deasphalted oil of the base oil (1), (2), (3), (4) or (5) (DAO);

(7) a mild hydrocracked oil of the base oil (6) (MHC); and

(8) a mixed oil of at least two selected from the base oils (1) to (7).

Preferred examples of the above described predetermined refining methodinclude hydrorefining such as hydrocracking and hydrofinishing; solventrefining such as furfural solvent extraction; dewaxing such as solventdewaxing and catalytic dewaxing; clay refining by acid clay, activatedclay, etc.; and chemical (acid or alkali) washing such as sulfuric acidwashing and caustic soda washing. In the present invention, theserefining methods may be carried out individually, or at least tworefining methods may be carried out in combination. When at least tworefining methods are combined, the order thereof is not restricted, andcan be suitably determined.

The following base oil (9) or (10) is especially preferable as thelubricant base oil of the present embodiment. The base oils (9) and (10)are obtained by carrying out a predetermined process on a base oilselected from the base oils (1) to (8), or on lubricant oil fractionsrecovered from any of the base oils (1) to (8):

(9) a hydrocracked base oil obtained by: hydrocracking a base oilselected from the base oils (1) to (8), or lubricant oil fractionsrecovered from any of the base oils (1) to (8); carrying out a dewaxingprocess such as solvent dewaxing and catalytic dewaxing on the productsthereof, or lubricant oil fractions recovered from the products thereofby distillation or the like; and optionally further distilling theproducts thereof after the dewaxing process; and

(10) a hydroisomerized base oil obtained by: hydroisomerizing a base oilselected from the base oils (1) to (8), or lubricant oil fractionsrecovered from any of the base oils (1) to (8); carrying out a dewaxingprocess such as solvent dewaxing and catalytic dewaxing on the productsthereof, or lubricant oil fractions recovered from the products thereofby distillation or the like; and optionally further distilling theproducts thereof after the dewaxing process. A catalytic dewaxingprocess is preferable as the dewaxing process.

When the above described lubricant base oil (9) or (10) is obtained, asolvent refining process and/or hydrofinishing process may be furthercarried out at a suitable stage if necessary.

A catalyst used for the above described hydrocracking orhydroisomerization is not restricted, but a hydrocracking catalystcomprising a metal having hydrogenating ability (such as at least onemetal in the VIa group and VIII group of the periodic table) supportedon a composite oxide having cracking activity (for example,silica-alumina, alumina-boria and silica zirconia), or on at least oneof the composite oxides in combination, bound by binders, as a support;or a hydroisomerization catalyst comprising at least one metal havinghydrogenating ability including at least one group VIII metal supportedon a support including zeolite (such as ZSM-5, zeolite beta, andSAPO-11) is preferably used. A hydrocracking catalyst andhydroisomerization catalyst may be used in combination by laminating ormixing or the like.

Reaction conditions of hydrocracking and hydroisomerization are notrestricted. Preferably, the hydrogen partial pressure is 0.1 to 20 MPa,the average reaction temperature is 150 to 450° C., LHSV is 0.1 to 3.0hr⁻¹, and the hydrogen/oil ratio is 50 to 20000 scf/b.

The kinematic viscosity of the lubricant base oil of the presentembodiment at 100° C. is 2.0 to 8.0 mm²/s, preferably no more than 5mm²/s, more preferably no more than 4.5 mm²/s, further preferably nomore than 4.4 mm²/s, and especially preferably no more than 4.3 mm²/s;and preferably no less than 3.0 mm²/s, more preferably no less than 3.5mm²/s, further preferably no less than 3.8 mm²/s, and especiallypreferably no less than 4.0 mm²/s. The kinematic viscosity of thelubricant base oil at 100° C. of more than 8.0 mm²/s might lead todeteriorated low-temperature viscosity properties of the lubricating oilcomposition, and to insufficient fuel efficiency. The kinematicviscosity thereof of less than 2.0 mm²/s might lead to insufficient oilfilm formation at lubricating points, which results in poor lubricity,and to large evaporation loss of the lubricating oil composition.

The kinematic viscosity of the lubricant base oil of the presentembodiment at 40° C. is preferably no more than 40 mm²/s, morepreferably no more than 30 mm²/s, further preferably no more than 25mm²/s, especially preferably no more than 22 mm²/s, and most preferablyno more than 20 mm²/s. On the other hand, the kinematic viscositythereof at 40° C. is preferably no less than 6.0 mm²/s, more preferablyno less than 8.0 mm²/s, further preferably no less than 10 mm²/s,especially preferably no less than 12 mm²/s, and most preferably no lessthan 14 mm²/s. The kinematic viscosity of the lubricant base oil at 40°C. of more than 40 mm²/s might lead to deteriorated low-temperatureviscosity properties of the lubricating oil composition, and toinsufficient fuel efficiency. The kinematic viscosity thereof of lessthan 6.0 mm²/s might lead to insufficient oil film formation atlubricating points, which results in poor lubricity, and to largeevaporation loss of the lubricating oil composition.

In this description, “kinematic viscosity at 40° C.” means kinematicviscosity at 40° C. specified by ASTM D-445.

The viscosity index of the lubricant base oil of the present embodimentis preferably no less than 100, more preferably no less than 110,further preferably no less than 120, especially preferably no less than125, and most preferably no less than 130. The viscosity index thereofof less than 100 tends to lead to not only deterioratedviscosity-temperature characteristics, thermal and oxidation stability,and anti-evaporation performance of the lubricating oil composition, butalso an increased friction coefficient, and tends to lead todeteriorated anti-wear properties. The viscosity index in thisdescription means a viscosity index measured conforming to JIS K2283-1993.

The density of the lubricant base oil of the present embodiment at 15°C. (

) is preferably no more than 0.860, more preferably no more than 0.850,further preferably no more than 0.840, and especially preferably no morethan 0.835. The density at 15° C. in this description means densitymeasured at 15° C., conforming to JIS K 2249-1995.

The pour point of the lubricant base oil of the present embodiment ispreferably no more than −10° C., more preferably no more than −12.5° C.,further preferably no more than −15° C., especially preferably no morethan −17.5° C., and most preferably no more than −20.0° C. The pourpoint beyond this upper limit tends to lead to deterioratedlow-temperature fluidity of whole of the lubricating oil composition.The pour point in this description means a pout point measuredconforming to JIS K 25269-1987.

The sulfur content in the lubricant base oil of the present embodimentdepends on the sulfur content in its raw material. For example, when asubstantially sulfur-free raw material, such as a synthetic waxcomponent obtained through Fischer-Tropsch reaction or the like, isused, a substantially sulfur-free lubricant base oil can be obtained.When a raw material containing sulfur, such as a slack wax obtainedthrough the process of refining the lubricant base oil, and a microwaxobtained through a wax refining process, is used, the sulfur content inthe obtained lubricant base oil is usually no less than 100 mass ppm. Inthe lubricant base oil of the present embodiment, in view of furtherimprovement of the thermal and oxidation stability and reduction of thesulfur content, the sulfur content is preferably no more than 100 massppm, more preferably no more than 50 mass ppm, further preferably nomore than 10 mass ppm, and especially preferably no more than 5 massppm.

The nitrogen content in the lubricant base oil of the present embodimentis preferably no more than 10 mass ppm, more preferably no more than 5mass ppm, and further preferably no more than 3 mass ppm. The nitrogencontent beyond 10 mass ppm tends to lead to deteriorated thermal andoxidation stability. The nitrogen content in this description means anitrogen content measured conforming to JIS K 2609-1990.

Preferably, % C_(P) of the lubricant base oil of the present embodimentis no less than 70, more preferably no less than 80, and furtherpreferably no less than 85; and usually no more than 99, preferably nomore than 95, and more preferably no more than 94. % C_(P) of thelubricant base oil under this lower limit tends to lead to deterioratedviscosity-temperature characteristics, thermal and oxidation stability,and friction properties, and further, to decreased effects of anadditive when the additive is incorporated to the lubricant base oil. %C_(P) of the lubricant base oil beyond this upper limit tends to lead todecreased solubility of an additive.

Preferably, % C_(A) of the lubricant base oil of the present embodimentis no more than 2, more preferably no more than 1, further preferably nomore than 0.8, and especially preferably no more than 0.5. % C_(A) ofthe lubricant base oil beyond this upper limit tends to lead todeteriorated viscosity-temperature characteristics, thermal andoxidation stability, and fuel efficiency.

Preferably, % C_(N) of the lubricant base oil of the present embodimentis no more than 30, more preferably no more than 25, further preferablyno more than 20, and especially preferably no more than 15. Preferably,% C_(N) of the lubricant base oil is no less than 1, and more preferablyno less than 4. % C_(N) of the lubricant base oil beyond this upperlimit tends to lead to deteriorated viscosity-temperaturecharacteristics, thermal and oxidation stability, and frictionproperties. % C_(N) thereof under this lower limit tends to lead todecreased solubility of an additive.

In this description, % C_(P), % C_(N) and % C_(A) mean percentage of theparaffin carbon number to all the carbon atoms, percentage of thenaphthene carbon number to all the carbon atoms, and percentage of thearomatic carbon number to all the carbon atoms, respectively, obtainedby the method conforming to ASTM D 3238-85 (n-d-M ring analysis). Thatis, the above described preferred ranges of % C_(P), % C_(N), and %C_(A) are based on values obtained according to the above method. Forexample, the value of % C_(N) obtained according to the above method canindicate more than 0 even if the lubricant base oil does not containnaphthenes.

The saturated content in the lubricant base oil of the presentembodiment is preferably no less than 90 mass %, preferably no less than95 mass %, and more preferably no less than 99 mass %, on the basis ofthe total mass of the lubricant base oil. The proportion of thecyclic-saturated content to the saturated content is preferably no morethan 40 mass %, preferably no more than 35 mass %, preferably no morethan 30 mass %, more preferably no more than 25 mass %, and furtherpreferably no more than 21 mass %. The proportion of the cyclicsaturated content to the saturated content is also preferably no lessthan 5 mass %, and more preferably no less than 10 mass %. The saturatedcontent and the proportion of the cyclic-saturated content to thesaturated content within these ranges can improve viscosity-temperaturecharacteristics, and thermal and oxidation stability. When an additiveis incorporated to the lubricant base oil, functions of the additive canbe brought out at a higher level while the additive is sufficientlystably dissolved and retained in the lubricant base oil. Further,friction properties of the lubricant base oil itself can be improved,and as a result, friction-reducing performance can be improved, whichleads to improvement of energy efficiency. In this description, thesaturated content means a value measured conforming to ASTM D 2007-93.

Any of similar methods from which the same results are obtained can beused for each of a method of separating the saturated content, and thecomposition analysis of e.g. the cyclic saturated content, and thenoncyclic saturated content. Examples thereof include the above methodspecified in ASTM D 2007-93, the method specified in ASTM D 2425-93, themethod specified in ASTM D 2549-91, methods using high performanceliquid chromatography (HPLC), and methods obtained by improving thesemethods.

The aromatic content in the lubricant base oil of the present embodimentis no more than 10 mass %, preferably no more than 5 mass %, morepreferably no more than 4 mass %, further preferably no more than 3 mass%, and especially preferably no more than 2 mass %, on the basis of thetotal mass of the lubricant base oil; and is preferably no less than 0.1mass %, more preferably no less than 0.5 mass %, further preferably noless than 1 mass %, and especially preferably no less than 1.5 mass %.The aromatic content beyond this upper limit tends to lead todeteriorated viscosity-temperature characteristics, thermal andoxidation stability, friction properties, moreover to deterioratedanti-evaporation performance and low-temperature viscosity properties,and further, when an additive is incorporated to the lubricant base oil,to decreased effects of the additive. Although the lubricant base oil ofthe present embodiment does not have to contain the aromatic content,the aromatic content of no less than this lower limit can furtherimprove solubility of an additive.

In this application, the aromatic content means a value measuredconforming to ASTM D 2007-93. The aromatic content usually includesalkylbenzenes, and alkylnaphthalenes; anthracenes, phenanthrenes, andalkylated compounds thereof; and compounds having four or more fusedbenzene rings, aromatic compounds having hetero atoms such as pyridines,quinolines, phenols, and naphthols.

A synthetic base oil may be used as the lubricant base oil of thepresent embodiment. Examples of the synthetic base oil includepoly-α-olefins, and hydrogenated products thereof; isobutene oligomers,and hydrogenated products thereof; isoparaffins; alkylbenzenes;alkylnaphthalenes; diesters (such as ditridecyl glutarate,bis(2-ethylhexyl) azipate, diisodecyl azipate, ditridecyl azipate, andbis(2-ethylhexyl) sebacate); polyol esters (such as trimethylolpropanecaprylate, trimethylolpropane pelargonate, pentaerythritol2-ethylhexanoate, and pentaerythritol pelargonate); polyoxyalkyleneglycols; dialkyl diphenyl ethers; polyphenyl ethers; and mixturesthereof, having a kinematic viscosity of 2.0 to 8.0 mm²/s at 100° C. andhaving an aromatic content of no more than 10 mass %. Among them,poly-α-olefins are preferable. Typical samples of poly-α-olefins includeoligomers and co-oligomers of C₂-C₃₂, preferably C₆-C₁₆ α-olefins (suchas 1-octene oligomers, decene oligomers, and ethylene-propyleneco-oligomers) and hydrogenated products thereof.

The method for producing a poly-α-olefin is not restricted. Examplesthereof include a method of polymerizing an α-olefin in the presence ofa polymerization catalyst such as a catalyst containing a complex ofaluminum trichloride or boron trifluoride, and water, alcohol (such asethanol, propanol, and butanol), a carboxylic acid or an ester.

The lubricant base oil of the present embodiment either may be composedof one base oil component, or may contain a plurality of base oilcomponents, as long as the base oil as a whole has a kinematic viscosityat 100° C. of 2.0 to 8.0 mm²/s and has an aromatic content of no morethan 10 mass %.

The content of the lubricant base oil of the present embodiment in thelubricating oil composition of the present invention is normally no lessthan 70 mass %, preferably no less than 75 mass %, and more preferablyno less than 80 mass %; and normally no more than 90 mass %, on thebasis of the total mass of the composition when the composition is amulti-grade oil. The content thereof is normally no less than 80 mass %,preferably no less than 85 mass %, and more preferably no less than 90mass %; and normally no more than 95 mass %, on the basis of the totalmass of the composition when the composition is a single-grade oil.

<(B) Metallic Detergent>

The lubricant base oil of the present invention contains (B1) a metallicdetergent overbased with calcium carbonate (hereinafter may be referredto as “component (B1)”) and (B2) a metallic detergent overbased withmagnesium carbonate (hereinafter may be referred to as “component (B2)”)as (B) a metallic detergent (hereinafter may be referred to as“component (B)”). Examples of the component (B) include phenatedetergents, sulfonate detergents and salicylate detergents. Thesemetallic detergents can be used alone or in combination.

Preferred examples of a phenate detergent include overbased salts ofalkaline earth metal salts of compounds having the structure of thefollowing formula (1). Examples of alkaline earth metals includemagnesium, barium, and calcium, Among them, magnesium and calcium arepreferable.

In the formula (1), R¹ is a C₆-C₂₁ linear or branched, saturated orunsaturated alkyl or alkenyl group; m is a polymerization degree,representing an integer of 1 to 10; A is a sulfide (—S—) group ormethylene (—CH₂—) group; and x is an integer of 1 to 3. R¹ may becombination of at least two different groups.

The carbon number of R¹ in the formula (1) is preferably 9 to 18, andmore preferably 9 to 15. The carbon number of R¹ of less than 6 mightlead to poor solubility in the base oil. On the other hand, the carbonnumber of R¹ beyond 21 makes the compound difficult to be produced, andmight lead to poor thermal stability.

The polymerization degree m in the formula (1) is preferably 1 to 4. Thepolymerization degree m within this range can improve thermal stability.

Preferred examples of a sulfonate detergent include alkaline earth metalsalts of alkyl aromatic sulfonic acids obtained by sulfonation ofalkylaromatics, or basic or overbased salts thereof. The weight-averagemolecular weight of the alkylaromatics is preferably 400 to 1500, andmore preferably 700 to 1300.

Examples of alkaline earth metals include magnesium, barium, andcalcium, Magnesium and calcium are preferable. Examples of alkylaromatic sulfonic acids include what is called petroleum sulfonic acidsand synthetic sulfonic acids, Examples of petroleum sulfonic acids hereinclude sulfonated products of alkylaromatics of lubricant oil fractionsderived from mineral oils, and what is called mahogany acid, which is aside product of production of white oils. Examples of synthetic sulfonicacids include a sulfonated product of alkylbenzene having a linear orbranched alkyl group, obtained by recovering side products in amanufacturing plant of alkylbenzene, which is a raw material ofdetergents, or by alkylating benzene with polyolefins, Another exampleof synthetic sulfonic acids includes a sulfonated product ofalkylnaphthalenes such as dinonylnaphthalene. A sulfonating agent usedwhen sulfonating these alkylaromatics is not limited. For example, afuming sulfuric acid or a sulfuric anhydride can be used as thesulfonating agent.

Preferred examples of a salicylate detergent include metallicsalicylates or basic or overbased salts thereof. Preferred examples ofmetallic salicylates here include compounds represented by the followingformula (2):

In the above formula (2), each R² is independently a C₁₄-C₃₀ alkyl oralkenyl group; M is an alkaline earth metal; and n is 1 or 2. M ispreferably calcium or magnesium. Preferably n is 1. When n=2, R² may becombination of different groups.

A preferred embodiment of a salicylate detergent can be an alkalineearth metal salicylate of the above formula (2) wherein n=1, or a basicor overbased salt thereof.

A method for producing alkaline earth metal salicylate is notrestricted, and known methods for producing monoalkylsalicylates can beused. For example, an alkaline earth metal salicylate can be obtainedby: reacting a metal base such as an oxide and hydroxide of an alkalineearth metal with a monoalkylsalicylic acid obtained by alkylating aphenol as a starting material with an olefin, and then carboxylating theresultant product with a carbonic acid gas or the like, or amonoalkylsalicylic acid obtained by alkylating a salicylic acid as astarting material with an equivalent of the olefin, or the like; onceconverting the above monoalkylsalicylic acid or the like to an alkalimetal salt such as a sodium salt and potassium salt, and then performingtransmetallation with an alkaline earth metal salt; or the like.

A method for obtaining an alkaline earth metal phenate, sulfonate, orsalicylate overbased with calcium carbonate or magnesium carbonate isnot limited. For example, it can be obtained by reacting an alkalineearth metal phenate, sulfonate, or salicylate with a base such ascalcium carbonate and magnesium carbonate in the presence of carbonicacid gas.

The metal ratio of the component (B) is a value calculated according tothe following formula; preferably no less than 1, more preferably noless than 2, and especially preferably no less than 3; and preferably nomore than 50, more preferably no more than 30, and especially preferablyno more than 10.

The metal ratio of the component (B)=the valence of the metal element inthe component (B)×the metal content in the component (B) (mol)/the soapgroup content of the component (B) (mol)

Examples of the component (B1) include calcium phenate detergents,calcium sulfonate detergents, calcium salicylate detergents, andcombination thereof, which are overbased with calcium carbonate.Preferably, the component (B1) contains at least a calcium salicylatedetergent.

Examples of the component (B2) include magnesium phenate detergents,magnesium sulfonate detergents, magnesium salicylate detergents, andcombination thereof, which are overbased with magnesium carbonate.Preferably, the component (B2) contains at least a magnesium salicylatedetergent, or a magnesium sulfonate detergent.

The content of the component (B1) in the lubricating oil composition issuch that the composition has the calcium content of no more than 1500mass ppm, and preferably has 1400 to 1500 mass ppm, on the basis of thetotal mass of the composition. The calcium content beyond 1500 mass ppmmakes LSPI easy to occur. The calcium content of no less than this lowerlimit makes it possible to maintain high detergency inside an engine,and to improve base number retainability.

The content of the component (B2) in the lubricating oil composition issuch that the composition has the magnesium content of no less than 300mass ppm, and preferably has 350 to 600 mass ppm, on the basis of thetotal mass of the composition. The magnesium content of no less thanthis lower limit can improve engine detergency while suppressing LSPI.The magnesium content of no more than this upper limit can suppressincrease of friction coefficients.

<(C) Molybdenum Friction Modifier (MoDTC)>

The lubricating oil composition of the present invention contains amolybdenum sulfide dithiocarbamate or a molybdenum oxysulfidedithiocarbamate as (C) a molybdenum friction modifier (hereinafter maybe referred to as “component (C)”). For example, a compound representedby the following formula (3) can be used as the component (C):

In the above general formula (3), R³ to R⁶ may be either the same ordifferent, and is a C₂-C₂₄ alkyl or C₆-C₂₄ (alkyl)aryl group, preferablya C₄-C₁₃ alkyl or C₁₀-C₁₅ (alkyl)aryl group. This alkyl group may be aprimary, secondary, or tertiary alkyl group, and may be linear orbranched. It is noted that “(alkyl)aryl group” means “aryl or alkylarylgroup”, In an alkylaryl group, the alkyl substituent may be in anyposition of the aromatic ring. Y¹ to Y⁴ are each independently a sulfuratom or oxygen atom, At least one of Y¹ to Y⁴ is a sulfur atom.

The content of the component (C) in the lubricating oil composition issuch that the composition has the molybdenum content of no less than 600mass ppm, and preferably no less than 700 mass ppm; and preferably nomore than 1000 mass ppm, more preferably no more than 900 mass ppm,further preferably no more than 850 mass ppm, and especially preferablyno more than 800 mass ppm, on the basis of the total mass of thecomposition. The molybdenum content of this lower limit or over canimprove fuel efficiency and LSPI suppression performance. The molybdenumcontent of this upper limit or below can improve storage stability ofthe lubricating oil composition.

<(D) Antioxidant>

The lubricating oil composition of the present invention preferablycontains an amine antioxidant and/or a phenol antioxidant as (D) anantioxidant (hereinafter may be referred to as “component (D)”). As theamine antioxidant, a known amine antioxidant such as alkylateddiphenylamine, alkylated phenyl-naphthylamine, phenyl-α-naphthylamine,and phenyl-β-naphthylamine can be used without any specific limitation.As the phenol antioxidant, a known phenol antioxidant such as2,6-di-tert-butyl-4-meth ylphenol (DBPC), and4,4′-methylenebis(2,6-di-tert-butylphenol) can be used without anyspecific limitation. When the lubricating oil composition of the presentinvention contains an antioxidant, the content thereof is usually 0.1 to5 mass % on the basis of the total mass of the composition.

The lubricating oil composition of the present invention preferablycontains an amine antioxidant as the component (D). When the lubricatingoil composition of the present invention contains an amine antioxidant,the content thereof is preferably 0.01 to 0.1 mass % in terms ofnitrogen on the basis of the total mass of the composition. The contentof the amine antioxidant in terms of nitrogen of this lower limit orover can further improve lifetime performance of the lubricating oil.The content thereof of this upper limit or below can suppress staininside an engine.

<(E) Zinc Dialkyldithiophosphate>

The lubricating oil composition of the present invention preferablycontains (E) a zinc dialkyldithiophosphate (ZnDTP; hereinafter may bereferred to as “component (E)”), For example, a compound represented bythe following formula (4) can be used as the component (E):

In the formula (4), R⁷ to R¹⁰ are independently a C₁-C₂₄ linear orbranched alkyl group, and may be combination of different groups. Thecarbon numbers of R⁷ to R¹⁰ are preferably no less than 3, preferably nomore than 12, and more preferably no more than 8. R⁷ to R¹⁰ may beprimary, secondary, or tertiary alkyl groups; and preferably primary orsecondary alkyl groups, or combination thereof. Preferably, the moleratio of primary alkyl group and secondary alkyl group (primary alkylgroup:secondary alkyl group) is 0:100 to 30:70. This ratio may be theintramolecular combination ratio of alkyl chains, or may be the mixingratio of ZnDTP having only primary alkyl groups and ZnDTP having onlysecondary alkyl groups. When secondary alkyl groups are major, fuelefficiency can be improved.

A method for producing the above zinc dialkyldithiophosphate is notlimited. For example, zinc dialkyldithiophosphate can be synthesized by:reacting alcohol(s) having an alkyl group corresponding to R⁷ to R¹⁰with phosphorus pentasulfide, to synthesize dithiophosphoric acid; andneutralizing the dithiophosphoric acid with zinc oxide.

When the lubricating oil composition of the present invention containsthe component (E), the content thereof is preferably 0.03 to 1.0 mass %on the basis of the total mass of the composition. The content of thecomponent (E) is preferably such that the phosphorus content in thecomposition is 750 to 800 mass ppm on the basis of the total mass of thecomposition. The phosphorus content in the composition of this lowerlimit or over can improve not only oxidation stability but also LSPIsuppression performance. The phosphorus content therein of this upperlimit or below makes it possible to avoid degradation of a base numberdue to hydrolysis of zinc dialkyldithiophosphate.

<(F) Corrosion Inhibitor or Metal Deactivator>

The lubricating oil composition of the present invention preferablycontains (F) a corrosion inhibitor or a metal deactivator (hereinaftermay be referred to as “component (F)”). Known corrosion inhibitors canbe used as the component (F) without any specific limitation, examplesof which include benzotriazole, tolyltriazole, thiadiazole, andimidazole compounds, and known metal deactivators such as imidazolines,pyrimidine derivatives, alkylthiadiazoles, mercaptobenzothiazoles,benzotriazoles and their derivatives, 1,3,4-thiadiazole polysulfide,1,3,4-thiadiazolyl-2,5-bis(dialkyl dithiocarbanate),2-(alkyldithio)benzimidazole, and β-(o-carboxybenzylthio) propionitrile.When the lubricating oil composition of the present invention containsthe component (F), the content thereof is usually 0.005 to 5 mass % onthe basis of the total mass of the composition.

The lubricating oil composition of the present invention preferablycontains a sulfur-containing compound as the component (F). Preferredexamples of a corrosion inhibitor or a metal deactivator that is asulfur-containing compound include thiadiazole. Use of asulfur-containing compound as the component (F) can further improve LSPIsuppression performance, and can effectively bring out friction-reducingperformance of the component (C), which is a molybdenum frictionmodifier. When the lubricating oil composition of the present inventioncontains a sulfur-containing compound as a corrosion inhibitor or ametal deactivator, the content thereof is usually no less than 0.01 mass%, preferably no less than 0.05 mass %, and more preferably no less than0.1 mass %; and usually no more than 1.0 mass %, preferably no more than0.5 mass %, and more preferably no more than 0.3 mass %.

The sulfur content in the lubricating oil composition is preferably 0.20to 0.30 mass %, and more preferably 0.23 to 0.28 mass %, on the basis ofthe total mass of the composition. The sulfur content therein of thislower limit or more can further improve LSPI suppression performance,and can effectively bring out friction-reducing performance of thecomponent (C), which is a molybdenum friction modifier. The sulfurcontent therein of this upper limit or below makes it possible tomaintain high detergency inside an engine.

<(G) Nitrogen-Containing Ashless Dispersant>

The lubricating oil composition of the present invention may contain anitrogen-containing ashless dispersant (hereinafter may be referred toas “component (G)”).

Examples of the component (C) include at least one compound selectedfrom the following (G-1) to (G-3):

(G-1) succinimide having at least one alkyl or alkenyl group in itsmolecule, or derivatives thereof (hereinafter may be referred to as“component (G-1)”);

(G-2) benzylamine having at least one alkyl or alkenyl group in itsmolecule, or derivatives thereof (hereinafter may be referred to as“component (G-2)”); and

(G-3) polyamine having at least one alkyl or alkenyl group in itsmolecule, or derivatives thereof (hereinafter may be referred to as“component (G-3)”).

The component (G-1) can be especially preferably used as the component(G).

In the component (G-1), examples of succinimide having at least onealkyl or alkenyl group in its molecule include compounds represented bythe following formula (5) or (6):

In the formula (5), R¹¹ is a C₄₀-C₄₀₀ alkyl or alkenyl group; hrepresents an integer of 1 to 5, preferably 2 to 4. The carbon number ofR¹¹ is preferably no less than 60, and preferably no more than 350.

In the formula (6), R¹² and R¹³ are independently C₄₀-C₄₀₀ alkyl oralkenyl group, and may be combination of different groups. R¹² and R¹³are especially preferably polybutenyl groups. In addition, i representsan integer of 0 to 4, preferably 1 to 3. The carbon number of R¹² andR¹³ is preferably no less than 60, and preferably no more than 350.

The carbon numbers of R¹¹ to R¹³ in the formulae (5) and (6) of theselower limits or over make it possible to obtain good solubility in thelubricant base oil. On the other hand, the carbon numbers of R¹¹ to R¹³of these upper limits or below can improve low-temperature fluidity ofthe lubricating oil composition.

The alkyl or alkenyl groups (R¹¹ to R¹³) in the formulae (5) and (6) maybe linear or branched. Preferred examples thereof include branched alkylgroups and branched alkenyl groups derived from oligomers of olefinssuch as propene, 1-butene, and isobutene, or from co-oligomers ofethylene and propylene. Among them, a branched alkyl or alkenyl groupderived from oligomers of isobutene that are conventionally referred toas polyisobutylene, or a polybutenyl group is most preferable.

Preferred number-average molecular weight of the alkyl or alkenyl groups(R¹¹ to R¹³) in the formulae (5) and (6) is 800 to 3500.

Succinimide having at least one alkyl or alkenyl group in its moleculeincludes so-called monotype succinimide represented by the formula (5),where a succinic anhydride terminates only one end of a polyamine chain,and so-called bistype succinimide represented by the formula (6), wheresuccinic anhydrides terminate both ends of a polyamine chain. Thelubricating oil composition of the present invention may include eithermonotype or bistype succinimide, and may include both of them as amixture.

A method for producing a succinimide having at least one alkyl oralkenyl group in its molecule is not limited. For example, suchsuccinimide can be obtained by: reacting an alkyl succinic acid or analkenyl succinic acid obtained by reacting a compound having a C₄₀-C₄₀₀alkyl or alkenyl group with maleic anhydride at 100 to 200° C., with apolyamine, Here, examples of polyamine include diethylenetriamine,triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine.

In the component (G-2), examples of benzylamine having at least onealkyl or alkenyl group in its molecule include compounds represented bythe following formula (7):

In the formula (7), R¹⁴ is a C₄₀-C₄₀₀ alkyl or alkenyl group; and jrepresents an integer of 1 to 5, preferably 2 to 4. The carbon number ofR¹⁴ is preferably no less than 60, and preferably no more than 350.

A method for producing the component (G-2) is not limited. An example ofsuch a method include: reacting a polyolefin such as propylene oligomer,polybutene, and ethylene-α-olefin copolymer, with phenol, to give analkylphenol; and then reacting the alkylphenol with formaldehyde, and apolyamine such as diethylenetriamine, triethylenetetramine,tetraethylenepentamine, and pentaethylenehexamine, by Mannich reaction.

In the component (G-3), examples of polyamine having at least one alkylor alkenyl group in its molecule include compounds represented by thefollowing formula (8):

R¹⁵—NH—(CH₂CH₂NH)_(k)—H  (8)

In the formula (8), R¹⁵ is a C₄₀-C₄₀₀ alkyl or alkenyl group; krepresents an integer of 1 to 5, preferably 2 to 4. The carbon number ofR¹⁵ is preferably no less than 60, and preferably no more than 350.

A method for producing the component (G-3) is not limited. An example ofsuch a method include: chlorinating a polyolefin such as propyleneoligomer, polybutene, and ethylene-α-olefin copolymer; and then reactingthe chlorinated polyolefin with ammonia, or a polyamine such asethylenediamine, diethylenetriamine, triethylenetetramine,tetraethylenepentamine, and pentaethylenehexamine.

Examples of derivatives in the components (G-1) to (G-3) include:

(i) an oxygen-containing organic compound-modified compound where a partor all of the residual amino and/or imino groups is/are neutralized oramidated by reacting the succinimide, benzylamine, or polyamine havingat least one alkyl or alkenyl group in its molecule (hereinafterreferred to as “the above described nitrogen-containing compound”) witha C₁-C₃₀ monocarboxylic acid such as fatty acids, a C₂-C₃₀polycarboxylic acid (such as ethanedioic acid, phthalic acid,trimellitic acid, and pyromellitic acid), an anhydride or ester thereof,a C₂-C₆ alkylene oxide, or a hydroxy(poly)oxyalkylene carbonate;

(ii) a boron-modified compound where a part or all of the residual aminoand/or imino groups is/are neutralized or amidated by reacting the abovedescribed nitrogen-containing compound with boron;

(iii) a phosphoric acid-modified compound where a part or all of theresidual amino and/or imino groups is/are neutralized or amidated byreacting the above described nitrogen-containing compound withphosphoric acid;

(iv) a sulfur-modified compound obtained by reacting the above describednitrogen-containing compound with a sulfur compound; and

(v) a modified compound obtained by two or more modifications selectedfrom oxygen-containing organic compound-modification,boron-modification, phosphoric acid-modification, andsulfur-modification, on the above described nitrogen-containingcompound.

Among the derivatives (i) to (v), using a boronated compound ofalkenylsuccinimide, especially a boronated compound of bistypealkenylsuccinimide can further improve the thermal stability of thelubricating oil composition.

The molecular weight of the component (C) is not specifically limited,but preferred weight-average molecular weight thereof is 1000 to 20000.

When the lubricating oil composition of the present invention containsthe component (C), the content thereof is, in terms of nitrogen on thebasis of the total mass of the lubricating oil composition, preferablyno less than 0.01 mass %, and more preferably no less than 0.03 mass %;and preferably no more than 0.15 mass %, more preferably no more than0.1 mass %, and especially preferably no more than 0.07 mass %. Thecontent of the component (C) of this lower limit or over cansufficiently improve anti-coking performance (thermal durability) of thelubricating oil composition. The content thereof of this upper limit orbelow makes it possible to maintain high fuel efficiency.

The boron content in the lubricating oil composition is, on the basis ofthe total mass of the lubricating oil composition, preferably no lessthan 0 mass ppm, more preferably no less than 100 mass ppm, andespecially preferably no less than 200 mass ppm; and preferably lessthan 400 mass ppm, more preferably no more than 350 mass ppm, andespecially preferably no more than 300 mass ppm. The boron content ofthis upper limit or below makes it possible to maintain high fuelefficiency, while keeping the ash content of the composition low.

<(H) Viscosity Index Improver>

The lubricating oil composition of the present invention preferablycontains (H) a viscosity index improver (hereinafter may be referred toas “component (H)”). Examples of the component (H) includenon-dispersant or dispersant poly(meth)acrylate viscosity indeximprovers, (meth)acrylate-olefin copolymers, non-dispersant ordispersant ethylene-α-olefin copolymers or hydrogenated productsthereof, polyisobutylene or hydrogenated products thereof, hydrogenatedstyrene-diene copolymers, styrene-maleic anhydride/ester copolymers, andpolyalkylstyrene.

The component (H) preferably contains a poly(meth)acrylate viscosityindex improver comprising 10-90 mol % of the structural unitsrepresented by the following general formula (9) (hereinafter may bereferred to as “viscosity index improver of the present embodiment”) onthe basis of the total monomer units in the polymer:

[In the general formula (9), R¹⁶ is hydrogen or a methyl group, and R¹⁷is a linear or branched chain hydrocarbon group having a carbon numberof 1 to 5.]

In the viscosity index improver of the present embodiment, the contentof the (meth)acrylate structural units represented by the generalformula (9) in the polymer is preferably 10 to 90 mol %, more preferablyno more than 80 mol %, and further preferably no more than 70 mol %; andmore preferably no less than 20 mol %, further preferably no less than30 mol %, and especially preferably no less than 40 mol %. The contentof the (meth)acrylate structural units represented by the generalformula (9) on the basis of the total monomer units of the polymerbeyond 90 mol % may lead to inferior solubility in the base oil,inferior improvement effect on viscosity-temperature characteristics,and inferior low-temperature viscosity characteristics. The contentunder 10 mol % may lead to inferior improvement effect onviscosity-temperature characteristics.

The viscosity index improver of the present embodiment may be acopolymer comprising another (meth)acrylate structural unit in additionto the (meth)acrylate structural unit represented by the general formula(9). Such a copolymer can be obtained by copolymerizing one or moremonomer(s) represented by the following general formula (10)(hereinafter referred to as “monomer (M-1)”), and a monomer other thanthe monomer (M-1).

[In the formula (10), R¹⁸ is a hydrogen atom or a methyl group, and R¹⁹is a linear or branched chain hydrocarbon group having a carbon numberof 6 to 18.]

Any monomer can be combined with the monomer (M-1). For example, amonomer represented by the following general formula (11) (hereinafterreferred to as “monomer (M-2)”) is preferable. A copolymer of themonomer (M-1) and the monomer (M-2) is a so-called non-dispersantpoly(meth)acrylate viscosity index improver,

[In the formula (11), R²⁰ is a hydrogen atom or a methyl group, and R²¹is a linear or branched chain hydrocarbon group having a carbon numberof 19 or more.]

R²¹ in the monomer (M-2) represented by the formula (11) is a linear orbranched chain hydrocarbon group having a carbon number of 19 or more asdescribed above, preferably a linear or branched chain hydrocarbon grouphaving 20 or more carbons, further preferably a linear or branched chainhydrocarbon group having 22 or more carbons, and more preferably abranched chain hydrocarbon group having 24 or more carbons. The upperlimit of the carbon number of the hydrocarbon group represented by R²¹is not restricted, but this hydrocarbon group is preferably a linear orbranched chain hydrocarbon group having 50,000 or less carbons, morepreferably a linear or branched chain hydrocarbon group having 500 orless carbons, further preferably a linear or branched chain hydrocarbongroup having 100 or less carbons, especially preferably a branched chainhydrocarbon group having 50 or less carbons, and most preferably abranched chain hydrocarbon group having 25 or less carbons.

One preferred example of the viscosity index improver of the presentembodiment is comb-shaped poly(meth)acrylate. Comb-shapedpoly(meth)acrylate here means a copolymer of the monomer (M-1) and themonomer (M-2), wherein the monomer (M-2) is a macromonomer including R²¹in the formula (11) having a number-average molecular weight (Mn) of1,000 to 50,000 (preferably 1,500 to 20,000, and more preferably 2,000to 10,000). Examples of such a macromonomer include a macromonomerderived from a hydrogenated product of a polyolefin obtained bycopolymerizing butadiene and isoprene.

In the viscosity index improver of the present embodiment, the polymermay comprise either only one kind of (meth)acrylate structural unitscorresponding to the monomer (M-2) represented by the general formula(11), or combination of two or more kinds thereof. The content of thestructural units corresponding to the monomer (M-2) represented by thegeneral formula (11) is, on the basis of the total monomer units of thepolymer, preferably 0.5 to 70 mol %, more preferably no more than 60 mol%, further preferably no more than 50 mol %, especially preferably nomore than 40 mol %, and most preferably no more than 30 mol %; andpreferably no less than 1 mol %, more preferably no less than 3 mol %,further preferably no less than 5 mol %, and especially preferably noless than 10 mol %, The content of the structural units corresponding tothe monomer (M-2) represented by the general formula (11) on the basisof the total monomer units of the polymer beyond 70 mol % may lead toinferior improvement effect on viscosity-temperature characteristics,and inferior low-temperature viscosity characteristics. The contentunder 0.5 mol % may lead to inferior improvement effect onviscosity-temperature characteristics.

One or more selected from a monomer represented by the following generalformula (12) (hereinafter referred to as “monomer (M-3)”), and a monomerrepresented by the following general formula (13) (hereinafter referredto as “monomer (M-4)”) is/are preferable as other monomers to becombined with the monomer (M-1). A copolymer of the monomer (M-1) andthe monomer (M-3) and/or (M-4) is a so-called dispersantpoly(meth)acrylate viscosity index improver. This dispersantpoly(meth)acrylate viscosity index improver may further contain themonomer (M-2) as constituting monomer.

[In the formula (12), R²¹ is a hydrogen atom or a methyl group, R²³ isan alkylene group having a carbon number of 1 to 18, E¹ is an amineresidue or heterocyclic residue having 1 to 2 nitrogen atoms, and 0 to 2oxygen atoms, and a is 0 or 1.]

Specific examples of an alkylene group having a carbon number of 1 to 18represented by R²³ include ethylene group, propylene group, butylenegroup, pentylene group, hexylene group, heptylene group, octylene group,nonylene group, decylene group, undecylene group, dodecylene group,tridecylene group, tetradecylene group, pentadecylene group,hexadecylene group, heptadecylene group, and octadecylene group (eachalkylene group may be either a linear or branched chain).

Specific examples of a residue represented by E¹ include dimethylaminogroup, diethylamino group, dipropylamino group, dibutylamino group,anilino group, toluidino group, quinolidino group, acetylamino group,benzoylamino group, morpholino group, pyrrolyl group, pyrrolino group,pyridyl group, methylpyridyl group, pyrrolidinyl group, piperidinylgroup, quinolyl group, pyrrolidonyl group, pyrrolidono group,imidazolino group, and pyrazino group,

[In the formula (13), R²⁴ is s a hydrogen atom or a hydrocarbon group,and E² is a hydrocarbon group, or an amine residue or heterocyclicresidue having 1 to 2 nitrogen atoms, and 0 to 2 oxygen atoms]

Specific examples of a group represented by E² include dimethylaminogroup, diethylamino group, dipropylamino group, dibutylamino group,anilino group, toluidino group, quinolidino group, acetylamino group,benzoylamino group, morpholino group, pyrrolyl group, pyrrolino group,pyridyl group, methylpyridyl group, pyrrolidinyl group, piperidinylgroup, quinolyl group, pirrolidonyl group, pyrrolidono group,imidazolino group, and pyrazino group.

Preferred specific examples of the monomers (M-3) and (M-4) includedimethylaminomethyl methacrylate, diethylaminomethyl methacrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate,2-methyl-5-vinylpyridine, morpholinomethyl methacrylate, morpholinoethylmethacrylate, N-vinylpyrrolidone, and mixtures thereof.

Although the copolymerization molar ratio of the copolymer of themonomer (M-1) and the monomers (M-2) to (M-4) is not restricted, monomer(M-1):monomers (M-2) to (M-4) is preferably approximately 20:80 to90:10, more preferably 30:70 to 80:20, and further preferably 40:60 to70:30.

The viscosity index improver of the present embodiment may be producedby any method. For example, they can be easily obtained by radicalsolution polymerization of the monomer (M-1) and/or (M-2), and one ormore selected from the monomers (M-3) to (M-4) under presence of apolymerization initiator such as benzoyl peroxide.

PSSI of the viscosity index improver of the present embodiment measuredby the diesel injector method is preferably no more than 40, morepreferably no more than 10, further preferably no more than 5,especially preferably no more than 3, and most preferably no more than1, The PSSI greater than 40 means low shear stability, and may lead todeteriorated fuel efficiency at the initial stage of use so as tomaintain a certain level of kinematic viscosity and HTHS viscosity afteruse. The lower limit of the PSSI of the viscosity index improver of thepresent embodiment is not restricted, but usually greater than 0. “PSSI”in this description means a permanent shear stability index of a polymercalculated based on data measured according to ASTM D 6278-02 (TestMethod for Shear Stability of Polymer Containing Fluids Using a EuropeanDiesel Injector Apparatus), conforming to ASTM D 6022-01 (StandardPractice for Calculation of Permanent Shear Stability Index).

The weight-average molecular weight (Mw) of the viscosity index improverof the present embodiment is usually 10,000 to 700,000, preferably noless than 20,000, more preferably no less than 50,000, furtherpreferably no less than 100,000, and especially preferably no less than120,000; and preferably no more than 500,000, more preferably no morethan 400,000, and further preferably no more than 300,000. When theweight-average molecular weight is under 10,000, not only viscosityindex improvement effect is small and fuel efficiency andlow-temperature viscosity characteristics deteriorate when the viscosityindex improver is dissolved in the lubricant base oil, but also the costmight rise. When the weight-average molecular weight is over 700,000,not only viscosity increase effect is too large, and fuel efficiency andlow-temperature viscosity characteristics deteriorate, but also shearstability, the solubility in the lubricant base oil, and storagestability deteriorate.

The ratio of the weight-average molecular weight to PSSI (Mw/PSSI) ofthe viscosity index improver of the present embodiment is preferably noless than 1.0×10⁴, more preferably no less than 2.0×10⁴, furtherpreferably no less than 5.0×10⁴, and especially preferably no less than8.0×10⁴. Mw/PSSI of less than 1.0×10⁴ may lead to deteriorated fuelefficiency and low temperature startability, that is, deterioratedviscosity temperature characteristics and low-temperature viscositycharacteristics.

The ratio of the weight-average molecular weight (Mw) to thenumber-average molecular weight (Mn) (Mw/Mn) of the viscosity indeximprover of the present embodiment is preferably no more than 4.0, morepreferably no more than 3.5, further preferably no more than 3.0,especially preferably no more than 2.0, and most preferably no more than1.5; and preferably no less than 1.0, more preferably no less than 1.05,and further preferably no less than 1.1. Mw/Mn beyond 4.0 leads todeteriorated solubility and deteriorated improvement effect of viscositytemperature characteristics, which may lead to failure to maintainsufficient storage stability and fuel efficiency.

The content of the component (H) inclusive of diluent in the lubricatingoil composition of the present invention is, on the basis of the totalmass of the composition, usually 0.1 to 30 mass %, preferably no lessthan 1 mass %, more preferably no less than 3 mass %, and furtherpreferably no less than 5 mass %; and preferably no more than 20 mass %,and more preferably no more than 15 mass %. The content of less than 0.1mass % may lead to deteriorated fuel efficiency, and insufficient lowtemperature characteristics. The content beyond 30 mass % may lead todeteriorated fuel efficiency and shear stability of the composition.

<Other Additives>

Other additives that are commonly used in lubricating oil can beincorporated in the lubricating oil composition of the present inventionaccording to its purpose in order to further improve its performance.Examples of such additives include additives such as friction modifiersother than the component (C), anti-wear agents (or extreme-pressureagents), anti-rust agents, demulsifiers, and defoaming agents.

Examples of friction modifiers other than the component (C) that can beused here include one or more friction modifier selected from ashlessfriction modifiers and organic molybdenum compounds other than thecomponent (C). The content of the friction modifier other than thecomponent (C) is preferably 0.01 to 2.0 mass % on the basis of the totalmass of the lubricating oil composition. Containing the frictionmodifier other than the component (C) can further improve fuelefficiency.

Examples of organic molybdenum compounds other than the component (C)include molybdenum dithiophosphate; complexes of molybdenum compounds(examples thereof include: molybdenum oxide such as molybdenum dioxideand molybdenum trioxide; molybdenum acids such as orthomolybdic acid,paramolybdic acid, sulfurized (poly)molybdic acid; molybdic acid saltssuch as metal salts and ammonium salts of these molybdic acids;molybdenum sulfides such as molybdenum disulfide, molybdenum trisulfide,molybdenum pentasulfide, molybdenum polysulfide; thiomolybdic acid;metal salts and amine salts of thiomolybdic acid; and molybdenum halidessuch as molybdenum chloride), and sulfur-containing organic compounds(examples thereof include: alkyl (thio)xanthate, thiadiazole,mercaptothiadiazole, thiocarbonate, tetrahydrocarbylthiuram disulfide,bis(di(thio)hydrocarbyldithiophosphonate) disulfide, organic(poly)sulfide, and sulfurized ester) or other organic compounds; andsulfur-containing organic molybdenum compounds such as complexes ofsulfur-containing molybdenum compounds such as the above describedmolybdenum sulfides, sulfurized molybdic acids, and alkenylsuccinimide.These organic molybdenum compounds may be either mononuclear molybdenumcompounds, or polynuclear molybdenum compounds such as binuclear ortrinuclear molybdenum compounds.

An organic molybdenum compound which does not contain sulfur as aconstituting element can be used as an organic molybdenum compound otherthan the component (C). Specific examples of an organic molybdenumcompound which does not contain sulfur as a constituting element includemolybdenum-amine complex, molybdenum-succinimide complex, molybdenumsalt of organic acids, and molybdenum salt of alcohols. Among them,molybdenum-amine complex, molybdenum salt of organic acids, andmolybdenum salt of alcohols are preferable.

When an organic molybdenum compound is used as the friction modifierother than the component (C), the content thereof is preferably 0.01 to2.0 mass % on the basis of the total mass of the composition. When anorganic molybdenum compound is used as the friction modifier other thanthe component (C), the molybdenum content in the lubricating oilcomposition is, on the basis of the total mass of the composition, noless than 600 mass ppm, and preferably no less than 700 mass ppm; andpreferably no more than 1000 mass ppm, more preferably no more than 900mass ppm, further preferably no more than 850 mass ppm, and especiallypreferably no more than 800 mass ppm as well. The content below thislower limit tends to lead to insufficient friction reducing effectdespite addition of the compound, and insufficient fuel efficiency, andthermal and oxidation stability of the lubricating oil composition. Onthe other hand, the content beyond this upper limit does not bring theeffect corresponding thereto, and tends to lead to deteriorated storagestability of the lubricating oil composition.

Any compound usually used as an ashless friction modifier forlubricating oil can be used as an ashless friction modifier withoutparticular limitation. Examples of ashless friction modifiers includecompounds each having one or more heteroatoms selected from oxygen,nitrogen, and sulfur in its molecule, and each having a carbon number of6-50. More specific examples thereof include ashless friction modifierssuch as amine compounds, fatty acid esters, fatty acid amides, fattyacids, aliphatic alcohols, aliphatic esters, urea compounds, andhydrazide compounds, each of which has at least one C₆-C₃₀ alkyl oralkenyl group, especially a linear alkyl group, a linear alkenyl group,a branched alkyl group, or a branched alkenyl group each having 6-30carbons, in the molecule.

When the lubricating oil composition contains an ashless frictionmodifier, the content thereof is preferably no less than 0.01 mass %,more preferably no less than 0.1 mass %, and further preferably no lessthan 0.3 mass %; and preferably no more than 2 mass %, more preferablyno more than 1 mass %, and especially preferably no more than 0.8 mass%, on the basis of the total mass of the lubricating oil composition.The content of an ashless friction modifier of less than 0.01 mass %tends to lead to insufficient friction reducing effect despite additionthereof. The content thereof beyond 2 mass % tends to inhibit effect ofanti-wear additives or the like, or to lead to deteriorated solubilityof additives.

Any anti-wear agents (or extreme-pressure agents) used for lubricatingoil can be used as an anti-wear agent (or an extreme-pressure agent)without particular limitation. Examples thereof include sulfur-based,phosphorus-based, and sulfur-phosphorus-based extreme pressure agents,and specifically, phosphorous esters, thiophosphorous esters,dithiophosphorous esters, trithiophosphorous esters, phoshphate esters,thiophosphate esters, dithiophosphate esters, trithiophosphate esters,amine salts thereof, metal salts thereof, derivatives thereof,dithiocarbamate, zinc dithiocarbamate, disulfides, polysulfides,sulfurized olefins, and sulfurized oils. Among them, addition of asulfur-based extreme-pressure agent, especially sulfurized oil ispreferable. When the lubricating oil composition contains an anti-wearagent (or extreme-pressure agent), the content thereof is preferably0.01 to 10 mass % on the basis of the total mass of the composition.

Examples of anti-rust agents include petroleum sulfonate,alkylbenzenesulfonate, dinonylnaphthalenesulfonate, alkenylsuccinateesters, and polyol esters. When the lubricating oil composition containsan anti-rust agent, the content thereof is preferably 0.01 to 10 mass %on the basis of the total mass of the composition.

Examples of demulsifiers include polyoxyalkylene glycol-based nonionicsurfactants such as polyoxyethylene alkyl ether, polyoxyethylenealkylphenyl ether, and polyoxyethylene alkylnaphthyl ether. When thelubricating oil composition contains a demulsifier, the content thereofis preferably 0.01 to 10 mass % on the basis of the total mass of thecomposition.

Examples of defoaming agents include silicone oil having kinematicviscosity at 25° C. of 1000 to 100,000 mm²/s, alkenylsuccinimidederivatives, esters of polyhydroxy aliphatic alcohol and long-chainfatty acid, methyl salicylate, and o-hydroxybenzylalcohol. When thelubricating oil composition contains a defoaming agent, the contentthereof is preferably 0.01 to 10 mass % on the basis of the total massof the composition.

<Lubricating Oil Composition>

The kinematic viscosity of the lubricating oil composition of thepresent invention at 100° C. is preferably 4.0 to 12 mm²/s, morepreferably no more than 9.3 mm²/s, further preferably no more than 8.2mm²/s, especially preferably no more than 7.1 mm²/s, and most preferablyno more than 6.8 mm²/s; and more preferably no less than 5.0 mm²/s,further preferably no less than 5.5 mm²/s, especially preferably no lessthan 6.1 mm²/s, and most preferably no less than 6.3 mm²/s. Thekinematic viscosity of the lubricating oil composition at 100° C. under4.0 mm²/s may lead to insufficient lubricity. The kinematic viscositythereof beyond 12 mm²/s may lead to insufficient low-temperatureviscosity and fuel efficiency.

The kinematic viscosity of the lubricating oil composition of thepresent invention at 40° C. is preferably 4.0 to 50 mm²/s, morepreferably no more than 40 mm²/s, further preferably no more than 35mm²/s, further preferably no more than 32 mm²/s, especially preferablyno more than 30 mm²/s, and most preferably no more than 28 mm²/s; andmore preferably no less than 15 mm²/s, further preferably no less than18 mm²/s, further more preferably no less than 20 mm²/s, especiallypreferably no less than 22 mm²/s, and most preferably no less than 25mm²/s. The kinematic viscosity of the lubricating oil composition at 40°C. under 4 mm²/s may lead to insufficient lubricity. The kinematicviscosity thereof beyond 50 mm²/s may lead to insufficientlow-temperature viscosity and fuel efficiency.

The viscosity index of the lubricating oil composition of the presentinvention is preferably 140 to 400, more preferably no less than 160,further preferably no less than 180, especially preferably no less than200, and most preferably no less than 210. The viscosity index of thelubricating oil composition under 140 might make it difficult to improvefuel efficiency while keeping the HTHS viscosity at 150° C., andfurther, to reduce the low-temperature viscosity (for example, at −35°C. that is measurement temperature of the CCS viscosity specified in theSAE viscosity grade 0W-X, known as viscosity grades of fuel-economyoil), When the viscosity index of the lubricating oil composition isbeyond 400, the evaporation loss might increase, and troubles due toinsufficient solubility of additives and compatibility with sealmaterials might occur.

The HTHS viscosity of the lubricating oil composition of the presentinvention at 100° C. is preferably no more than 5.5 mPa·s, morepreferably no more than 5.0 mPa·s, further preferably no more than 4.9mPa·s, especially preferably no more than 4.8 mPa·s, and most preferablyno more than 4.6 mPa·s; and preferably no less than 3.5 mPa·s, morepreferably no less than 4.0 mPa·s, further preferably no less than 4.4mPa, and especially preferably no less than 4.5 mPa·s. In thisdescription, the HTHS viscosity at 100° C. means high temperature highshear viscosity at 100° C., specified in ASTM D4683. The HTHS viscosityat 100° C. under 3.5 mPa·s may lead to insufficient lubricity. The HTHSviscosity at 100° C. beyond 5.5 mPa·s may lead to insufficientlow-temperature viscosity and fuel efficiency.

The HTHS viscosity of the lubricating oil composition of the presentinvention at 150° C. is no more than 2.7 mPa·s, preferably no more than2.65 mPa·s, and especially preferably no more than 2.35 mPa·s; andpreferably no less than 1.95 mPa·s, more preferably no less than 2.1mPa·s, further preferably no less than 2.2 mPa·s, and especiallypreferably no less than 2.25 mPa·s. In this description, the HTHSviscosity at 150° C. means high temperature high shear viscosity at 150°C., specified in ASTM D4683. The HTHS viscosity at 150° C. under 1.95mPa·s may lead to insufficient lubricity. The HTHS viscosity at 150° C.beyond 2.7 mPa·s may lead to insufficient fuel efficiency.

The ratio (X₁₀₀/X₁₅₀) of the HTHS viscosity at 100° C. (X₁₀₀) to theHTHS viscosity at 150° C. (X₁₅₀) of the lubricating oil composition ofthe present invention is preferably no more than 2.0. The ratio of theHTHS viscosity X₁₀₀/X₁₅₀ of no more than 2.0 makes it possible toachieve high fuel efficiency while maintaining anti-wear performance.The lower limit of the ratio of the HTHS viscosity X₁₀₀/X₁₅₀ is notspecifically restricted, but is preferably no less than 1.8. The ratioof the HTHS viscosity X₁₀₀/X₁₅₀ of no less than 1.8 makes it possible tomaintain high base oil viscosity, which is advantageous in view ofevaporation loss and anti-wear performance.

The evaporation loss of the lubricating oil composition according to thepresent invention is, as NOACK evaporation loss at 250° C., preferablyno more than 20 mass %, further preferably no more than 15 mass %, andespecially preferably no more than 14 mass %. When the NOACK evaporationloss is beyond 20 mass %, the evaporation loss of the lubricating oil islarge, which causes viscosity increase and the like, and is notpreferable. The NOACK evaporation loss in the present description isevaporation loss of the lubricating oil measured conforming to ASTM D5800. The lower limit of the NOACK evaporation loss of the lubricatingoil composition at 250° C. is not restricted, but normally no less than5 mass %.

The inventors have examined operation of a turbocharged testing engineunder operation conditions such that LSPI easily occurs, and have foundthat occurrence frequency of LSPI has a negative correlation with anautoignition point in differential scanning calorimetry (DSC) under anair or oxygen atmosphere at a pressure of 10 atm.

In the engine test, so as to exclude influence of deposits formed in acombustion chamber, preconditioning operation at partial load wascarried out at 4000 rpm for 30 minutes, and thereafter the throttleposition, rotation speed, injection timing, air-fuel ratio, etc. werechanged to operation conditions such that LSPI easily occurs (throttle:fully opened, rotation speed: 1800 rpm), Then the number of occurrenceof LSPI within 1 hour was measured by means of combustion pressuresensors attached to each cylinder of the engine.

In the DSC measurements, 5 mg of an engine oil sample was heatedtogether with a standard material, under an air or oxygen atmosphere ata pressure of 10 atm, at a temperature increase rate of 10 K/min, toobtain a function of difference of input energy against temperature. Inthe obtained function, an autoignition point was determined as thelowest temperature at which an exothermic peak begins.

FIG. 1 is a scatter diagram in which the occurrence frequency of LSPI inthe engine test is plotted against the autoignition point of the engineoil sample used in the engine test measured in the DSC measurement underan air atmosphere at a pressure of 10 atm (hereinafter may be referredto as “DSC (10 atm air atmosphere) autoignition point”). It can be seenthat when the DSC (10 atm air atmosphere) autoignition point increases,for example, from 260° C. to 270° C., the occurrence frequency of LSPIdecreases to about 1/7. While the graph of FIG. 1 shows correlationbetween the DSC (10 atm air atmosphere) autoignition point and theoccurrence frequency of LSPI, the correlation between the autoignitionpoint in DSC measurement under an oxygen atmosphere at a pressure of 10atm (hereinafter may be referred to as “DSC (10 atm oxygen atmosphere)autoignition point”) and the occurrence frequency of LSPI is consideredto be higher than this.

The DSC (10 atm oxygen atmosphere) autoignition point of the lubricatingoil composition of the present invention is preferably no less than 213°C., more preferably no less than 215° C., further preferably no lessthan 217° C., and especially preferably no less than 220° C. The upperlimit thereof is not specifically restricted, but usually no more than300° C., and typically no more than 280° C. The DSC (10 atm oxygenatmosphere) autoignition point of this lower limit or over caneffectively suppress the occurrence frequency of LSPI.

The lubricating oil composition of the present invention preferably hasa parameter r_(s) represented by the following mathematical formula (1)of no less than 1.08, more preferably no less than 1.10, furtherpreferably no less than 1.15, and especially preferably no less than1.20. The parameter r_(s) is preferably no more than 3.00, morepreferably no more than 2.00, and especially preferably no more than1.50.

r _(s)=([S]+[Mo]+[Zn])/([Mg]+2×[Ca])  (1)

(in the mathematical formula (1), [S] represents the sulfur contentderived from additives (unit: mass ppm); [Mo] represents the molybdenumcontent of the composition (unit: mass ppm); [Zn] represents the zinccontent of the composition (unit: mass ppm); [Mg] represents themagnesium content of the composition (unit: mass ppm); and [Ca]represents the calcium content of the composition (unit: mass ppm).)

The parameter r_(s) within the above described range makes it possibleto achieve well-balanced fuel efficiency, engine detergency, and LSPIsuppression performance.

The lubricating oil composition of the present invention preferably hasa parameter r_(s)′ represented by the following mathematical formula (2)of no less than 1.00, more preferably no less than 1.02, furtherpreferably no less than 1.05, especially preferably no less than 1.10,and most preferably no less than 1.15. The parameter r_(s)′ ispreferably no more than 2.50, more preferably no more than 2.00, andespecially preferably no more than 150.

r _(s)′=([S]′+[Mo]+[Zn])/([Mg]+2×[Ca])  (2)

(in the mathematical formula (2), [S]′ represents the sulfur contentderived from additives other than any sulfonate detergent (unit: massppm); [Mo] represents the molybdenum content of the composition (unit:mass ppm); [Zn] represents the zinc content of the composition. (unit:mass ppm); [Mg] represents the magnesium content of the composition(unit: mass ppm); and [Ca] represents the calcium content of thecomposition (unit: mass ppm).)The parameter r_(s)′ within the above described range makes it possibleto achieve well-balanced fuel efficiency, engine detergency, and LSPIsuppression performance.

<Method for Suppressing LSPI of Internal Combustion Engine>

The method for suppressing LSPI of an internal combustion engineaccording to the second aspect of the present invention includes a stepof operating an internal combustion engine, while lubricating a cylinderof the internal combustion engine by means of the above describedlubricating oil composition according to the first aspect of the presentinvention. In the method for suppressing LSPI of the present invention,the lubricating oil composition of the present invention is used for atleast lubrication of the cylinder, and a portion of the internalcombustion engine other than the cylinder may be lubricated togetherwith the cylinder by means of the lubricating oil composition of thepresent invention. Known lubricating oil supply mechanisms can beemployed for lubricating the cylinder of the internal combustion engineby means of the composition, without particular limitation, Lubricatingthe cylinder of the internal combustion engine by means of thecomposition of the present invention effectively suppresses LSPI in theinternal combustion engine.

EXAMPLES

Hereinafter the present invention will be more specifically describedbased on. Examples and Comparative Examples. It is noted that thepresent invention is not limited to these examples.

Examples 1 to 8 and Comparative Examples 1 to 5

Each of the lubricating oil compositions of the present invention(Examples 1 to 8) and for comparison (Comparative examples 1 to 5) wasprepared using the following base oil and additives. In Tables, “inmass%” means mass % on the basis of the total mass of the base oil, “mass %”means mass % on the basis of the total mass of each composition, and“mass ppm” means mass ppm on the basis of the total mass of eachcomposition.

(Base Oil)

O-1: Group III base oil, kinematic viscosity (100° C.): 4.15 mm²/s,aromatic content: 0.2 mass %

(Metallic Detergent)

B1-1: CaCO₃-overbased Ca salicylate, Ca content: 8.0 mass %, metalratio: 3.0, base number (perchloric acid method): 225 mgKOH/g, sulfurcontent: 0.0 mass %

B1-2: CaCO₃-overbased Ca sulfonate, Ca content: 12.75 mass %, basenumber (perchloric acid method): 325 mgKOH/g, sulfur content: 2.0 mass %

B2-1: MgCO₃-overbased Mg sulfonate, Mg content: 9.3 mass %, base number(perchloric acid method): 400 mgKOH/g, sulfur content: 2.0 mass %

(Molybdenum Friction Modifier)

C-1: molybdenum (oxy)sulfide dithiocarbamate, alkyl group: combinationof C₈ and C₁₃, Mo content: 10.0 mass %, sulfur content: 10.8 mass %

(Antioxidant)

D-1: amine antioxidant, nitrogen content: 3.6 mass %

D-2: phenol antioxidant

(Zinc Dithiophosphate)

E-1: zinc dialkyldithiophosphate (alkyl group: secondary C₆, Zn content:9.25 mass %, phosphorus content: 8.5 mass %, sulfur content: 17.6 mass%)

(Ashless Dispersant)

G-1: polybutenylsuccinimide, bis-type, number-average molecular weightof polybutenyl group: 1300, nitrogen content: 1.75 mass %

G-2: boronated polybutenylsuccinimide, bis-type, number-averagemolecular weight of polybutenyl group: 1300, nitrogen content: 1.5 mass%, boron content: 0.78 mass %

(Viscosity Index Improver)

H-1: polymethacrylate viscosity index improver, weight-average molecularweight: 500,000, PSSI: 5

(Other Sulfur-Containing Additives)

I-1: alkyldithiothiadiazole, sulfur content: 36.0 mass %

I-2: sulfurized olefin, sulfur content: 46.0 mass %

TABLE 1 Examples Comparative examples 1 2 1 2 3 4 (A) Base oil O-1inmass % 100 100 100 100 100 100 (B) Metallic detergent B1-1 mass % —1.88 — — 2.50 — B1-2 mass % 1.18 — 1.57 — — 1.18 B2-1 mass % 0.36 0.36 —1.45 — 0.36 (C) Mo Friction modifier C-1 mass % 0.70 6.70 0.70 0.70 0.70— (E) Zinc dithiophosphats E-1 mass % 0.94 0.94 0.94 0.94 0.94 6.94 (G)Ashless dispersant G-1 mass % 3.29 3.29 3.29 3.29 3.29 3.29 Content ofelement Ca mass ppm 1500 1500 2000 0 2000 1500 Mg mass ppm 340 340 01350 0 340 Mo mass ppm 700 700 700 700 700 0 B mass ppm 0 0 0 0 0 0 Pmass ppm 800 800 800 800 800 800 Zn mass ppm 870 870 870 870 870 870Sulfur content derived from additives mass % 0.27 0.25 0.27 0.27 0.240.20 Sulfur content derived from additives mass % 0.24 0.24 0.24 0.240.24 0.17 (other than any sulfonate detergent) HTT290 deposit mg 6.7 3.217.2 2.3 24.1 30.5 SRV friction coefficient (100° C.) 0.060 0.084 0.0630.088 0.052 0.165 r_(s) 1.28 1.22 1.07 3.16 0.99 0.86 r_(s)′ 1.19 1.190.99 2.94 0.99 0.77

TABLE 2 Examples 3 4 5 6 (A) Base oil O-1 inmass % 100.0 100.0 100 100.0(B) Metallic detergent B1-1 mass % 1.75 1.75 1.75 1.75 B2-1 mass % 0.380.38 0.65 0.65 (C) Mo Friction modifier C-1 mass % 0.70 0.70 0.70 0.70(D) Antioxidant D-1 mass % 1.00 1.00 1.00 1.00 D-2 mass % 0.50 0.50 0.500.50 (E) Zinc dithiophosphate E-1 mass % 0.94 0.94 0.94 6.94 (G) Ashlessdispersant G-2 mass % 3.30 3.30 3.30 3.30 (H) Viscosity index improverH-1 mass % 6.0 10.0 6.0 10.0 Content of element Ca mass ppm 1400 14001400 1400 Mg mass ppm 350 350 600 600 Mo mass ppm 700 700 700 700 B massppm 260 260 260 260 P mass ppm 800 800 800 800 Zn mass ppm 870 870 870870 Sulfur content derived from additives 0.25 0.25 0.25 0.25 Sulfurcontent derived from additives mass % 0.24 0.24 0.24 0.24 (other thanany sulfonate detergent) Kinematic viscosity (40° C.) mm²/s 25.6 27.825.7 27.9 Kinematic viscosity (100° C.) mm²/s 6.4 7.7 6.4 7.7 Viscosityindex 217 269 217 269 HTHS viscosity (100° C.) mPa · s 4.5 4.8 4.6 5.0HTHS viscosity (150° C.) mPa · s 2.3 2.6 2.3 2.6 HTT290 deposit mg 0.21.2 0.3 0.4 SRV friction coefficient (100° C.) 0.057 0.055 0.053 0.053r_(s) 1.29 1.29 1.20 1.20 r_(s)′ 1.26 1.26 1.17 1.17

TABLE 3 Examples Comparative examples 1 7 8 4 5 (A) Base oil O-1 inmass% 100 100 100 100 100 (B) Metallic detergent B1-1 mass % — — — — — B1-2mass % 1.18 1.18 1.18 1.18 1.18 B2-1 mass % 0.36 0.36 0.36 0.36 0.36 (C)Mo friction modifier C-1 mass % 0.70 0.70 0.70 — 0.70 (E) Zincdithiophosphate E-1 mass % 0.94 0.94 0.94 0.94 — (G) Ashless dispersantG-1 mass % 3.29 3.29 3.29 3.29 3.29 (I) other sulfur-containingadditives I-1 mass % — 0.10 — — — I-2 mass % — — 0.10 — — Content ofelement Ca mass ppm 1500 1500 1500 1500 1500 Mg mass ppm 340 340 340 340340 Mo mass ppm 700 700 700 0 700 B mass ppm 0 0 0 0 0 P mass ppm 800800 800 800 0 Zn mass ppm 870 370 870 870 0 Sulfur content derived fromadditives 0.27 0.31 0.32 0.20 0.11 Sulfur content derived from additivesmass % 0.24 0.28 0.29 0.17 0.08 (other than any sulfonate detergent) DSCautoignition point ° C. 220 222 223 211 212 r_(s) 1.28 1.40 1.43 0.860.54 r_(s)′ 1.19 1.31 1.34 0.77 0.45

(Evaluation of Lubricating Oil Compositions)

For each of the lubricating oil compositions of Examples 1 to 6 andComparative examples 1 to 4, the amount of deposits in a hot tube test(HTT290 deposit) was measured, and a friction coefficient (SRV frictioncoefficient) was measured by means of an SRV friction testing machine.For each of the lubricating oil compositions of Examples 3 to 6, HTHSviscosities at 100° C. and 150° C., kinematic viscosities at 100° C. and40° C., and a viscosity index were further measured. The results areshown in Tables 1 and 20 For each of the lubricating oil compositions ofExamples 1, 7 and 8, and Comparative examples 4 and 5, a DSC (10 atmoxygen atmosphere) autoignition point was also measured. The results areshown in Table 3. Measurement methods were as follows:

(1) HTT290 deposit: a hot tube test was carried out at 290° C.,conforming to JPI-5S-55-99, to measure the weight of deposits (unit: mg)adhering to the inner wall surface of a tube having a predetermined boreand length. Less deposits means higher engine detergency.

(2) SRV friction coefficient: a cylinder-on-disk test was carried out ta temperature of 100° C. at a load of 400 N at an amplitude of 1.5 mm ata frequency of 50 Hz by means of an SRV reciprocating friction weartesting machine (manufactured by Optimol Instruments), to measure afriction coefficient.

(3) HTHS viscosity: measured conforming to ASTM D-4683.

(4) kinematic viscosity: measured conforming to ASTM D-445.

(5) viscosity index: measured conforming to JIS K 2283-1993.

(6) DSC autoignition point: differential scanning calorimetry wascarried out under an oxygen atmosphere at a pressure of 10 atm at atemperature increase rate of 10° C./min by means of a pressuredifferential scanning calorimeter (manufactured by TA Instruments), todetermine an autoignition point as a temperature at which the peaksbegan. A higher autoignition point means less LSPI occurrence frequency.

INDUSTRIAL APPLICABILITY

The lubricating oil composition of the present invention has improvedLSPI suppression performance, and is superior in engine detergency andfuel efficiency. Thus, the lubricating oil composition of the presentinvention can be preferably used for lubrication of turbochargedgasoline engines which tend to suffer LSPI, especially for lubricationof turbocharged direct-injection engines.

1. A lubricating oil composition for an internal combustion enginecomprising: (A) a lubricant base oil having a kinematic viscosity at100° C. of 2 to 8 mm²/s and having an aromatic content of no more than10 mass %; (B) a metallic detergent comprising: (B1) a metallicdetergent overbased with calcium carbonate; and (B2) a metallicdetergent overbased with magnesium carbonate; and (C) a molybdenumsulfide dithiocarbamate or a molybdenum oxysulfide dithiocarbamate,wherein the composition has a calcium content of 1400 to 1500 mass ppmon the basis of the total mass of the composition; the composition has amagnesium content of 350 to 600 mass ppm on the basis of the total massof the composition; the composition has a molybdenum content of no lessthan 600 mass ppm on the basis of the total mass of the composition; thecomposition has a boron content of 200 to 300 mass ppm on the basis ofthe total mass of the composition; the composition has an HTHS viscosityat 150° C. of no more than 2.7 mPa·s, and the composition has aparameter r_(s)′ represented by the following mathematical formula (2)of 1.15 to 1.50:r _(s)′=([S]′+[Mo]+[Zn])/([Mg]+2×[Ca])  (2) wherein in the mathematicalformula (2), [S]′ represents a sulfur content derived from additivesother than any sulfonate detergent (unit: mass ppm); [Mo] represents themolybdenum content of the composition (unit: mass ppm); [Zn] representsa zinc content of the composition (unit: mass ppm); [Mg] represents themagnesium content of the composition (unit: mass ppm); and [Ca]represents the calcium content of the composition (unit: mass ppm). 2.The lubricating oil composition according to claim 1, comprising: (D) anamine antioxidant and/or a phenol antioxidant.
 3. The lubricating oilcomposition according to claim 1, comprising: (D) an amine antioxidant,wherein the composition has a ratio (X₁₀₀/X₁₅₀) of an HTHS viscosity at100° C. (X₁₀₀) to the HTHS viscosity at 150° C. (X₁₅₀) of no more than2.0.
 4. (canceled)
 5. The lubricating oil composition according to claim3, wherein the composition has the molybdenum content of 700 to 800 massppm on the basis of the total mass of the composition.
 6. Thelubricating oil composition according to claim 3, comprising: (E) a zincdialkyldithiophosphate, wherein the composition has a sulfur content of0.20 to 0.30 mass % on the basis of the total mass of the composition.7. The lubricating oil composition according to claim 3, wherein thecomposition has the ratio (X₁₀₀/X₅₀) of the HTHS viscosity at 100° C.(X₁₀₀) to the HTHS viscosity at 150° C. (X₁₅₀) of 1.8 to 2.0.
 8. Amethod for suppressing LSPI of an internal combustion engine, the methodcomprising: operating an internal combustion engine, while lubricating acylinder of the internal combustion engine by means of the lubricatingoil composition as in claim 1.